The present invention relates to printed circuit boards, and more particularly, to multilayer printed circuit boards for mounting and interconnecting electronic devices, such as LSI (large scale integration) and VLSI (very large scale integration) devices in a data processing system.
Multilayer printed circuit boards are generally manufactured by bonding stacks or sheets of substrates of insulating material having printed conductor or wiring patterns on their faces. The printed conductor patterns may be formed on each layer separately, and then the layers may be assembled and the printed conductor patterns on the layers electrically interconnected by employing plated-through holes or the like. Plated-through holes are usually formed by drilling the assembled layers or boards and plating the entire length of the drilled hole with copper or another conductor to effect the interconnection.
Alternatively, multilayer printed circuit boards have been formed by starting with a first substrate, forming a conductor pattern on that substrate, forming a second substrate directly over the first substrate and forming a conductor pattern on the second substrate, and continuing thereon until the desired number of layers have been formed.
Two common difficulties often arise in the design and manufacture of multilayer printed circuit boards that are used in mounting and interconnecting LSI and VLSI devices. Because of the large amount of circuitry within and the large number of external leads on these devices, there is often difficulty in trying to provide, during the design of the multilayer board, adequate electrical paths or connections between layers without using significant amounts of circuit board area which might otherwise be used to route printed conductor patterns. Where plated-through holes provide the electrical connections between layers, a hole of sufficient size must be drilled or otherwise made through each of the layers so that there can be enough plating material used to form an adequate electrical connection between all of the desired layers.
Another difficulty that often arises is that in multilayer boards, especially boards for mounting and connecting LSI and VLSI devices and thus having a large number of conductive patterns on the layers, an improper connection or other defect on any one of the layers makes the entire multilayer board unusable. Since the fabrication of a multilayer board is generally expensive both in design and production costs, the necessity of discarding a board because of a defect or a circuit design change on only one layer further increases an already high cost.
In U.S. Pat. No. 3,780,352, issued to James O. Redwanz on Dec. 18, 1973, there is disclosed a multilayer printed circuit board comprised of thin, flexible sheets for connecting semiconductor chips in an integrated circuit package. The Redwanz patent does not, however, disclose a printed circuit board which might be used, for example, in a data processing system where a number of electronic devices are mounted on the board for electrical interconnection, nor does it deal with solving the difficulty when a defect or a design change occurs with respect to only one of the layers.
In U.S. Pat. No. 3,028,573, issued to Rudolph O. Stoehr on Apr. 3, 1962, there is disclosed a multilayered printed circuit board with the layers secured together by nuts and bolts. The circuit board in Stoehr could thus be disassembled by removing the nuts and bolts. However, unlike modern multilayer printed circuit boards, the Stoehr printed circuit board includes insulating layers between the layers having the conductor patterns, and uses removable pins for providing interlayer electrical connections. The printed circuit board in Stoehr would not be suitable for the complex and significant number of interconnections required in printed circuit boards used for mounting and interconnecting LSI and VLSI devices in data processing systems.